Image intensifier tubes are used in night vision devices to amplify light and allow a user to see images in very dark conditions. Night vision devices typically include a lens to focus light onto the light receiving end of an image intensifier tube and an eyepiece at the other end to view the enhanced imaged produced by the image intensifier tube.
Modern image intensifier tubes use photocathodes. Photocathodes emit electrons in response to being exposed to photons from an image. The electrons are produced in a pattern that replicates the original scene. The electrons from the photocathode are accelerated towards a microchannel plate. A microchannel plate is typically manufactured from lead glass and has a multitude of channels, each one operable to produce a cascade of secondary electrons in response to an incident electron.
Therefore, photons impinging on the photocathode produce electrons which are then accelerated to a microchannel plate where a cascade of secondary electrons are produced. These electrons are accelerated towards a phosphorous screen, where their collisions with the screen produces an image of the original scene.
A drawback to this approach is that the electrostatic fields in the image intensifier are not only effective in accelerating electrons from the photocathode to the microchannel plate and from the microchannel plate to the screen, but also moves any positive ions back to the photocathode at an accelerated velocity. Current image intensifiers have a high indigenous population of positive ions. These are primarily due to gas ions in the tube, including in the microchannel plate and the screen. These include both positive ions and chemically active neutral atoms. When these ions strike the photocathode, they can cause both physical and chemical damage. This leads to short operating lives for image intensifiers.
To overcome this problem, an ion barrier film can be placed on the input side of the microchannel plate. This ion barrier is able to block the ions from the photocathode.
One drawback of the ion barrier is that it reduces the signal to noise ratio of the image intensifier. This is due to the fact that the barrier is detrimental to ion transport.
Therefore, current image intensifiers require an ion barrier since current manufacturing techniques fail to remove enough gas molecules. But the presence of the ion barrier film reduces the signal to noise ratio. What is needed is an unfilmed (i.e. without ion barrier film) microchannel plate that has sufficient gas ions removed such that an image intensifier manufactured with such a microchannel plate has a usable life.